FIGS. 12 (A) and 12 (B) show a conventional polishing apparatus for polishing a wafer (substrate). Referring to FIGS. 12 (A) and 12 (B), according to the conventional polishing apparatus, a wafer 2 is polished through the steps of dropping a droplet of a slurry, which contains an abrasive agent and which is fed from a slurry feed means 6, on a polishing pad 1 adhered to a rotatable polishing table 3, pressing the wafer 2 rotated by a spindle 7 against the polishing pad. In order to remove debris and the like which clog traps (grooves) formed on the surface of the polishing pad 1, conditioning of a polishing pad (called as to "Ex-SITU conditioning") is performed by using a diamond disc 5 installed on a conditioning drive means 4 during the interval between polishing steps (runs).
Conventionally, conditioning conditions have been determined by practicing a pilot operation before advancing an actual polishing step of polishing a wafer to be changed into a product. Explaining more in detail, according to the prior method, a conditioning condition is set as follows. Many pilots (blank wafers) are polished changing the conditioning time. The thickness of each pilot is measured after a given time of polishing. When the pilot thickness coincides with the set thickness, the corresponding conditioning time is taken as a conditioning condition. In case of polishing wafers belonging to the same lot group or the same patterned group, the above pilot procedure by using one blank wafer per several ten pieces of lots is taken, and the conditioning time is determined on the result of this procedure.